1. Field of the Invention
The present invention relates to a method of preventing the overflow of cells, each including multi-media information in a communication network, based on an ATM (Asynchronous Transfer Mode).
2. Description of the Related Art
Recently, an ATM (Asynchronous Transfer Mode) has been put to practical use as a communication method of transmitting/receiving data at high speed. According to an ATM mode, user devices linked to a ring network load data on a packet having a fixed length and going round the ring to communicate. A packet is called a cell.
A plurality of ring networks are connected to each other by a cell switch (hereafter called a switch), and a cell can be transmitted from one network to another network by a switch. A switch has a plurality of input/output ports, and a single ring is connected to one of the input/output ports. To define broadly, a switch may have one of the following two structures. One of them is an input buffer type switch which has a buffer at the input side, and the other is an output buffer type switch which has a buffer at the output side.
When using an input buffer type switch, cells from input ports await transmission in the input buffer. Bandwidth in a single output port is shared by traffic from a plurality of input ports. If the number of cells in the input buffer increases in such a manner that causes an overflow, cells will be abandoned.
When using an output buffer type switch, cells from a plurality of input ports are switched by the switch before being transmitted to the respective output buffer. Subsequently, the output buffer transmits a cell to the ring network. If the traffic amount to an output buffer is great as compared to the bandwidth of the respective output port so that the output buffer is likely to overflow, some adjustment such as cell abandonment will be performed. Thus, when using an output buffer type switch, traffic from a plurality of input ports concentrate on a single output port, so that the total traffic amount could exceed the bandwidth of the output port. As a result, the output buffer overflows, and cells are abandoned. It is generally perceived that an output buffer type switch performs better than an input buffer type switch in terms of controlling overflow.
Abandonment of cells could be avoided by making the bandwidth large enough to assure the peak speed of cells as for each traffic. However, if the bandwidth is thus set for traffic of LAN data whose peak speed is great but traffic density is small, the throughput of a switch will be very small. To enhance the throughput of a switch, bandwidth must be set to assure the average speed rather than the peak speed if the quality of data is not highly requested. Even in this case, cells must be abandoned if a plurality of input ports happen to transmit cells simultaneously. In the prior art, the problem of cell abandonment has been solved by having the destination terminal identify abandonment of a cell, and request its re-transmission.
Japanese Laid-open Patent Application No. 4-291851 proposes "a method of notifying node state in a loop type LAN" (hereafter called prior art 1) to reduce re-transmissions of cells. In this method, when a node is busy, it notifies other nodes of its busy state by means of a busy notice cell. Accordingly, transmission to the node ceases. If controlling input/output ports based on this method, cell abandonments and re-transmissions will be expectedly reduced. Therefore, abandonment of cells by a switch can be partly prevented.
Also, a switch has to enable communications among a plurality of users linked to a single input/output port. Electronic information communication institute technical research report SSE 91-95, "Evaluation of GFC (Generic Flow Control) protocol in ATM-UNI (User Network Interface)" (hereafter called prior art 2) proposes a method of controlling the transmission amount from a plurality of user terminals to a network.
The basic operation of a user terminal in the prior art 2 is described. A plurality of user devices and a network controller are connected in a ring in the logical sense, and input/output of cells is controlled. The limit on the number of cells which can be transmitted to the ring, and a counter for counting the number of cells which have been transmitted are provided to each user terminal beforehand. One cycle represents the state of a user terminal in which the user terminal can transmit cells each carrying its own data. And a user terminal can transmit cells in one cycle up to the limit. A cell is structured into two parts: an area indicating the cell is a rest cell and an area indicating every user terminal has completed its transmissions. A user terminal completes its transmissions when the limit is reached or no cells to be transmitted exist.
When the network controller detects that every user completes transmissions, it sends a reset cell in the network which resets the counter. Upon each reset of the counter, a next cycle starts. When a reset cell is detected in the ring, a next cycle also starts. Consequently, flow control is fair to users in one cycle.
However, prior art 2 set forth in the above, re-transmission of a cell is requested after it was abandoned. Therefore, even when an output buffer is in an overflow state, cells could be transmitted to it. Further, re-transmission increases traffic, which makes cell abandonment more likely. As a result, the reliability of a system itself designed to be tolerant with cell abandonment is deteriorated.
Even the prior art 1 which aims at overcoming the above problem has another problem of control delay because the transmitting terminal has to fabricate a busy notice cell and the receiving terminal has to analyze a cell. Fabricating a busy notice cell and analyzing a cell are processed at a level higher than an ATM Adaptation Layer (AAL), and implemented by software.
Not only with a switch, but also with a line concentrator which has a plurality of low-speed input ports and converting traffics from them into one high-speed communication path (ATM MUX: Multiplexer), the same problem remain unsolved.